Think of this in terms of magnitude or size. A true ion is something like the sodium cation (Na+) or the chloride anion (Cl-). There is a full charge on the ion that is a whole integer value (1, 2, 3, etc.). A molecular dipole is not a full charge on an atom like you find in an ion. A dipole is a partial charge. If two atoms covalently bonded together experience a dipole, that means that one of the atoms will have a partial positive charge and the other atom will have a partial negative charge. This is because the electron density in the bond is shifted toward the the more electronegative atom. But the important point here is that a dipole is merely a partial charge, whereas an ion is a full charge. This is why they use the phrase "molecular dipole are only a fraction of the size of the charges on an ion."